Hui Su

Room temperature slow and fast light in quantum-dot semiconductor optical amplifiers

We demonstrate room temperature optically and electrically controllable group delay using population oscillation in a quantum-dot (QD) semiconductor optical amplifier (SOA). A reduction of the group index up to 10% with a bandwidth of 13 GHz is achieved under different configurations of injection current and optical pump intensity. Our theoretical results based on population pulsation agree well with experimental data. We extract the linewidth enhancement factor and effective carrier diffusion coefficient of the QD SOA. We also observe slow light when the injection current is increased.

Variable optical delay using population oscillation and four-wave-mixing in semiconductor optical amplifiers

We investigate variable optical delay of a microwave modulated optical beam in semiconductor optical amplifier/absorber waveguides with population oscillation (PO) and nearly degenerate four-wave-mixing (NDFWM) effects. An optical delay variable between 0 and 160 ps with a 1.0 GHz bandwidth is achieved in an InGaAsP/InP semiconductor optical amplifier (SOA) and shown to be electrically and optically controllable. An analytical model of optical delay is developed and found to agree well with the experimental data. Based on this model, we obtain design criteria to optimize the delay-bandwidth product of the optical delay in semiconductor optical amplifiers and absorbers.

Room-temperature slow light with semiconductor quantum-dot devices

We demonstrate room-temperature slow light that is electrically and optically controllable by using a quantum-dot (QD) semiconductor optical amplifier (SOA) at zero and low bias below the transparency current. The absorption spectrum of the QD SOA exhibits a spectral dip with a corresponding group-index dispersion and group delay owing to coherent population oscillation caused by the interaction of pump and probe laser light near resonance of the first heavy-hole-conduction-state transition. At an optical pump power of approximately 0.3 mW inside the single-mode waveguide without current injection, a group-index change of 3.0 with a bandwidth of 2 GHz was measured. This group-index change can be controlled by injection of electrical current and by changing the optical pump power.

Slow Light Based on Coherent Population Oscillation in Quantum Dots at Room Temperature

We develop a model for slow light based on the coherent population oscillation of quantum dots at room temperature. With the absorption dip and corresponding variation of the refractive index due to the coherent population beating induced by the pump and probe signal, quantum dots can be used as a slow-light medium. Our theoretical model matches the experimental results very well. We also experimentally and theoretically demonstrate that both the forward-bias injection current and reverse-bias voltage can change the group index in a semiconductor quantum-dot waveguide. Our results indicate that quantum-dot devices can be potentially used as electrically and optically controllable slow light devices

Comparison of linewidth enhancement factor between p-doped and undoped quantum-dot lasers

The optical linewidth enhancement factor (LEF) of a p-doped quantum-dot (QD) laser is measured below threshold and compared with a theoretical calculation. The optical gain, refractive index, and LEF are well matched with our theoretical model when the thermal effect is isolated by an additional pulse current measurement of the LEF. We also theoretically calculate the LEF of an undoped QD Fabry-Pe/spl acute/rot (FP) laser assuming that the structure of the undoped FP QD laser is the same as that of the p-doped QD FP laser except the p-type doping. The changes in modal gain and refractive index due to the respective QD ground and excited states are calculated. Based on the theoretical results, we show that the LEF of the p-doped QD laser is smaller than that of the undoped QD laser due to the reduced transparency carrier density.

Optical and electrical control of slow light in p-doped and intrinsic quantum-dot electroabsorbers

Room temperature p-doped and intrinsic quantum-dot semiconductor electroabsorption modulator is utilized as an optical group delay. Electrical (reverse voltage and forward current, below transparency) and optical controllable delay is realized by means of counterpropagating coherent population oscillation. Because of carrier dynamics, the experimental observations show bandwidth increase with reverse voltage and dependence of group delay on background absorption. Moreover, experimental observations show maximum group delay at a particular pump intensity, which is attributed to inhomogeneous contributions and saturation effects in quantum dots.

Electrically and optically controllable optical delay in a quantum-well semiconductor optical amplifier

An optical delay variable between 0 to 161 ps with GHz bandwidth is demonstrated in an InGaAsP quantum-well semiconductor optical amplifier and modeled by population oscillation and nearly degenerate four-wave-mixing effects for the first time.

Room temperature variable slow light using semiconductor quantum dots

We demonstrate slow light at room temperature using a quantum-dot semiconductor optical amplifier with a controllable delay by forward and reverse bias below the transparency current level.

Variable Slow Light Using Coherent Population Oscillation in Quantum-Dot Electro-Absorption Modulator

Room temperature quantum-dot semiconductor electro-absorption modulator is utilized as an optical group delay. Electrical (reverse and forward bias below transparency current level) and optical tunable buffer is realized by means of counter-propagating coherent population oscillation.

Slow Light Using Semiconductor Quantum Wells and Quantum Dots for Future Optical Networks

where ng is the group index of the optical frequency. A large and positive slope increases the group index and reduces the group velocity of the optical signal. Previous experiments of slow light were performed using the electromagnetically induced transparency (EIT) effects in systems with a long decoherence time, e.g., atomic gas [1] at very low temperatures. Although a slowdown factor up to 10 could be achieved, the experimental conditions and setups are not practical for system applications. Slow-light devices based on semiconductors are preferred because of the easy integration with other optoelectronic components. The radiative lifetime in semiconductors is a proper time scale for slow light. The nanosecond radiative lifetime corresponds to a gigahertz bandwidth and is suitable for practical applications. To utilize the radiative lifetime, a pump-probe setup sensing the population oscillation (PO) of the energy levels in the material system has been demonstrated [2][3]. We will describe slow light using the PO effects from heavy-hole (HH) excitons in quantum wells (QWs). This corresponds to the PO in an absorption medium. Extensions of PO to quantum dot (QD) gain media bring about interesting applications for active devices at room temperature and will also be addressed.